Change: rework GLOG chunk to be more like the rest

Basically it is very similar to Vehicles, where there first is
a type field, followed by data of that type. So this commit makes
it looks like how Vehicles solved that.
This removes a lot of custom "keeping track of length" stuff.

src/gamelog.h

@@ -13,7 +13,7 @@
 #include "newgrf_config.h"
 
 /** The actions we log. */
-enum GamelogActionType {
+enum GamelogActionType : uint8 {
 	GLAT_START,        ///< Game created
 	GLAT_LOAD,         ///< Game loaded
 	GLAT_GRF,          ///< GRF changed